Dynamic motion sensors (“motion sensors” or “sensors”) are transducers used for measuring acceleration. Dynamic motion sensors usually are sensitive along a single axis. This is what is referred to as a single axis or uni-axial dynamic motion sensor. For example, acceleration magnitude is measured by means of an external instrument or measuring system sensing the electrical output of the accelerometer. The accelerometer output is usually a voltage waveform proportional to the magnitude of the applied acceleration. Similarly, dynamic motion sensing devices such as velocimeters and dynamically responding displacement transducers output a voltage waveform or generate current proportional to velocity.
The process of calibrating an accelerometer or other motion sensor comprises applying a known measured or calculated motion to the dynamic motion sensor to be tested (the Sensor Under Test or the SUT), as determined by a high accuracy reference (REF) sensor. This is what is referred to as the back-to-back method. The method of back-to-back accelerometer calibration is covered under an ISO standard (ISO16063-21: Vibration Calibration by Comparison to a Reference Transducer) and will only be summarized here.
The SUT and REF devices are both attached to a device capable of imparting harmonic motion at significant displacement. One such device is depicted in FIG. 1 and is referred to as a long-stroke shaker or long-stroke vibrator. During the calibration, the reference transducer, which has previously been calibrated by other means such as laser interferometer (as described in ISO16063-11) is used to measure the applied acceleration. Both accelerometers output a time-varying electrical signal proportional to the magnitude of the acceleration to which the device is subjected. This time varying electrical signal may take the form of a voltage proportional to the magnitude, but it may also be in the form of a current, or charge proportional to the magnitude of acceleration. In the case of a current or charge output sensor, a signal conditioning unit is utilized in order to convert the current or charge into the appropriate voltage signal for the analog to digital converter used to digitize and record the acceleration magnitude signal. Calibration software executing on the microprocessor or computer connected to the REF or SUT is then responsible for calculating the sensitivity of the accelerometer to be calibrated by multiplying the sensitivity of the reference sensor by the ratio of the electrical signal magnitudes of the two devices. The ratio of SUT acceleration to REF acceleration is calculated and this ratio used to calculate the voltage sensitivity of the SUT accelerometer.
The REF accuracy and resolution is the limiting factor in the accuracy of the calibration operation. At lower frequencies, typically 10 Hz or lower, using an accelerometer or other type of motion sensor as a reference sensor limits system calibration accuracy due to the fact that the acceleration decreases as the square of frequency for a given displacement. This is what is commonly referred to as stroke limitation, and for that reason the industry typically utilizes shakers capable of generating at least 6″ of peak-to-peak (pk-pk) displacement for use in low frequency calibrations.
It should be noted that the ISO016063 standard suggests using either a method of root mean square (“RMS”) calculation or a spectral method of calculation of the magnitude of acceleration at the frequency of interest. The typical method used in the prior art has been to utilize an RMS method, but this is rapidly changing to the spectral method as it is capable of providing a superior result in terms of noise rejection thus improving the overall calibration accuracy.
Thus, there exists in the field a problem with the known method of calibrating dynamic motion sensors at low frequencies using a motion sensor as a reference or baseline standard.